The present invention relates generally to a method for joining laminated materials. More specifically, the present invention relates to a method for performing the joining together of two laminated materials including layers of orientation-stretched plastic material. The present invention also relates to a casing of plastic material joined together in accordance with the method.
It is known that in connection with packages a polyester material of good mechanical characteristics is used which, moreover, can be substantially improved when the laminated material is orientation-stretched. In order to utilize in full the good tensile strength characteristics of the material e.g. in packages containing pressurized contents (beer, refreshing drinks, etc.), the material should be biaxially oriented, i.e. in two directions at right angles to one another so that full use can be made of the material to absorb the tensile stresses emanating from the contents. The orientation of the material comes about by stretching the material at a temperature between 70.degree.-90.degree. C. either simultaneously or in two separate operations in two directions at right angles to one another. By this procedure, in addition to the molecular orientation aimed at and the substantially improved tensile strength capacity, a reduction of the thickness of the material in proportion to the stretching is also achieved. A normal orientation-stretching in biaxially oriented polyester material brings about a stretching of the material of 4 to 5 times in two directions at right angles to one another, which represents a surface enlargement of the material of 16 to 25 times. At the same time as the tensile strength characteristics of the polyester material are substantially improved, a crystallization of the polyester material also takes place, as a result of which the material cannot be heat-sealed. If the material is heated a shrinkage of the material will occur as the material endeavors to return to the form it had before the orientation-stretching. This characteristic, namely that the oriented polyester material cannot be heat-sealed, has greatly limited the possibility of using it for the manufacture of packages and the material has been used chiefly in the field of packaging for packages which are formed only by blowing or deep-drawing and do not contain any sealing joints.
However, in recent times modified polyester materials have been produced which can be stretched to a substantial degree without any crystallization or, more correctly, with only partial crystallization of the material. Such a modified polyester is a glycol-modified polyester which in general is designated PETG. The glycol-modified polyester can be readily laminated to ordinary polyester material, preferably by a co-extrusion operation, and, since the material is very closely akin, the adhesion obtained between the polyester layer and the PETG layer is so good that it is hardly possible to separate the two laminated layers. When the laminate is orientation-stretched the polyester layer crystallizes, while the modified polyester layer (PETG layer) crystallizes considerably more slowly and is heat-sealable at least up to an orientation stretching of 20-25 times. This means that an orientation-stretched laminate can also be heat-sealed, but now the problem arises, that a heating of the material brings about a shrinkage of the heated area. This shrinkage can be avoided in principle if during the heating and sealing operations the material is kept tightly pressed together, so that a shrinking movement of the material is rendered impossible, in this case, however, the orientation in the heated area is lost and the material consequently loses its strength in this area.
What is wanted, therefore, is a joining together which is mechanically strong but can be performed without the laminated material being heated so that it shrinks or so that the molecular orientation is lost.
The method according to the present invention is a joining together which solves the problem. In the method in accordance with the present invention a narrow strand of molten plastic material of the same or similar type as that which constitutes the material layers intended for joining together is applied to one or both the laminated materials along the intended joining area. The two material layers intended for joining together are then pressed together along the joining area while they take up between them the molten strand of plastic material. When heat is transferred from the plastic strand to the adjoining surfaces of the material layers intended to be joined together, a fusing together of material takes place between the surfaces and the plastic strand applied.